powercap/drivers/dtpm: Add hierarchy creation

The DTPM framework is available but without a way to configure it.

This change provides a way to create a hierarchy of DTPM node where
the power consumption reflects the sum of the children's power
consumption.

It is up to the platform to specify an array of dtpm nodes where each
element has a pointer to its parent, except the top most one. The type
of the node gives the indication of which initialization callback to
call. At this time, we can create a virtual node, where its purpose is
to be a parent in the hierarchy, and a DT node where the name
describes its path.

In order to ensure a nice self-encapsulation, the DTPM subsys array
contains a couple of initialization functions, one to setup the DTPM
backend and one to initialize it up. With this approach, the DTPM
framework has a very few material to export.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220128163537.212248-3-daniel.lezcano@linaro.org